With image-acquisition devices that employ an image-acquisition element based on the X-Y address system, for example, a CMOS image sensor or the like, when video images are acquired under illumination, in which the brightness thereof changes in accordance with the power-supply frequency, such as under a fluorescent lamp, flicker occurs in the form of a lateral stripe pattern with dark portions and bright portions periodically appearing in the vertical direction of a frame image. Thus, there are various known methods of decreasing the flicker components occurring in the image in this way by applying image processing to the flicker components (for example, see Patent Literatures 1 to 3).
With the image-processing method described in Patent Literature 1, in an image-acquisition device in which an image-acquisition element thereof has different photocharge accumulation times for individual lines, output signal levels of individual pixels in 3n (n is a positive integer) continuous frames are divided by the average of the output signal levels of the individual pixels is these 3n continuous frames. Then, the gains by which the output signal levels the individual pixels in the 3n continuous frames are amplified are controlled so as to be proportional to the inverse of the divided values. By doing so, the flicker components generated due to electric discharge when using illumination light are decreased.
Alternatively, the total output signal levels of individual pixel groups, each of which is formed of a plurality of pixels in the 3n continuous frames, are divided by the average of the total output signal levels of the plurality of pixel groups in the 3n continuous frames, and the gains by which the output signal levels of the individual pixels in the 3n continuous frames are amplified are controlled so as to be proportional to the inverse of the divided values.
In addition, with regard to the image-processing method described in Patent Literature 2, in addition to the basic configuration described in Patent Literature 1, Patent Literature 2 describes a method of achieving temporal stability of the calculated flicker components by using an infinite-impulse-response low-pass filter and a method of controlling the cyclic coefficient of the infinite-impulse-response low-pass filter on the basis of information about the movement between frames. With the method of Patent Literature 1, large fluctuations occur in the calculated flicker components when large movements occur among the 3n continuous frames; however, with the method of Patent Literature 2, it is assumed that misdetection and over correction of the flicker components can be prevented by using an infinite-impulse-response low-pass filter that works in association with the movement information.
Furthermore, in Patent Literature 3, in addition to the basic configuration described in Patent Literature 1, it is assumed that, even in the case in which the acquired image shows extremely large movements among the 3n continuous frames or even in the case in which the phases of the flicker components change due to changes in the power-supply frequency or the image-acquisition frame rate, it is possible to achieve excellent suppression of flicker by applying, after appropriate adjustments, the phases of flicker components calculated by using past frames to current frames.